In general, every electrical power supply system must be protected against short circuits in the system. Automatic fuses are provided for this purpose in the field of private building installation. In industry, in particular for electrical installations or machines, this protection is achieved by protective devices in the form of circuit breakers which are opened via appropriate tripping units in the event of a short circuit.
In the event of a short circuit, protective devices such as these first of all limit the current flowing in order to protect both the electrical installations (cables, busbars, etc.) and the connected loads (machines, etc.). The current limiting reduces the dynamic loads (resulting from the square of the current) and the thermal loads (resulting from the integral of the square of the current) on an electrical installation in the event of a short circuit. The load depends on the response time of the switch, which, as far as possible, should be less than 0.5 ms. The response time depends in particular on the time (self response time) which the electronic evaluation unit requires to identify a short circuit.
The problem of long short-circuit identification times (much more than 100 μs) of electronic tripping units is also particularly evident from characteristic diagrams of circuit breakers. If one compares the switching-off characteristics of two circuit breakers having an identical mechanical design and different tripping units (electronic and thermo-magnetic), then, particularly in the case of high short-circuit currents, a circuit breaker with an electronic tripping unit has a considerably longer switching-off time. This entails high loading of the installation to be protected on account of the relatively long action time, and also has an adverse effect on the switching capacity of the circuit breakers themselves as a result of the high amount of energy converted in the switch.
In many electronic releases, a short circuit is identified by comparing the instantaneous value of the current flowing with a threshold value, and this could take a relatively long time, depending on the nature of the short circuit.
In order to reduce the time before identification of a short circuit, it has already been proposed that the gradient of the current curve (first time derivative of the current) be considered as an alternative criterion. This type of tripping responds only to the gradient of the current curve, but not to the current level that is reached. Initially, this criterion was applied only to shorten the switching-off times of quick-action direct-current switches, since the relationship between the rate of current rise, the short-circuit current and the voltage in single-phase and polyphase systems is dependent on the time of the short circuit and on the power factor in the power supply system.
The combination of both criteria, the instantaneous value of the current and the current gradient, is proposed in DE 36 42 136 C2. For this purpose a sensor, for example a Rogowski converter, is used to determine the current gradients in every main current path. This is linked to formation of digital value pairs comprising a current gradient and an instantaneous value of the current, which are compared with predetermined threshold values. This comparison is preferably carried out using a locus curve criterion. The disadvantage of the algorithm described in DE 36 42 136 C2 is that it allows switching processes to be assessed only without an initial current. Changes in the power factor, for example caused by large machines being connected or disconnected, or other changes in the current, can thus themselves lead to tripping even though they are still within the permissible range.
DE 015 88 096 A uses the sum of the first and second derivatives of the current as a criterion for short-circuit identification. The second derivative is in this case produced by an LC circuit from the first derivative of the current, for which reason the signal is subject to a compensation element. Furthermore, because of the LC circuit, the signal is at a relatively low power level, making its evaluation more difficult. DE 015 88 096 A does not provide any details as to how to determine the response value g and how to determine the factors x and y.